Method and apparatus for providing stable voltage to analytical system

ABSTRACT

An electrochemical cell has two terminals. One of the terminals is connected to a pulse-width-modulated (PWM) power supply and to a voltmeter. The other terminal is connected to circuitry capable of switching between amperometric and potentiometric measurement modes. A sequence of successive approximations permits selection of a PWM duty cycle giving rise to a desired voltage at the terminal connected with the power supply. In this way a stable excitation voltage is supplied to the cell even in the face of supply voltage instability or drift or instability in electronics coupled with the cell.

BACKGROUND

It is not easy to make repeatable and accurate measurements inanalytical systems such as consumer devices using an electrochemicalcell. Many constraints contribute to the difficulty of this task. Theconsumer device must be light in weight, small and reliable. The pricecannot be too high. The device may be running on a new battery or an oldone, and the user cannot be relied upon to perform manual calibrationsteps. The repeatability and accuracy of the measurements must bepreserved even in the face of temperature changes and user decisionssuch as whether or not to use a display backlight.

SUMMARY OF THE INVENTION

An electrochemical cell has at least two terminals. One of the terminalsis connected to a pulse-width-modulated (PWM) power supply and to avoltmeter. Another of the terminals is connected to circuitry capable ofswitching between amperometric and potentiometric measurement modes. Asequence of successive approximations permits selection of a PWM dutycycle giving rise to a desired voltage at the terminal connected withthe power supply. In this way a stable excitation voltage is supplied tothe cell even in the face of supply voltage instability or drift orinstability in electronics coupled with the cell.

DESCRIPTION OF THE DRAWING

The FIGURE shows an exemplary circuit according to the invention.

DETAILED DESCRIPTION

The FIGURE shows an analytical system 39 with an electrochemical cell22. The analysis is performed under the control of a microcontroller 21.The microcontroller 21 has a pulse-width-modulated output 27 as well asanalog inputs 28, 32 and 29. The analog inputs 28, 32, 29 are (in anexemplary embodiment) connected by means of a multiplexer internal tothe microcontroller 21 to an analog-to-digital converter also internalto the microcontroller 21. PWM signal 27 controls transistors 25, 26which, through filter 24, develop a voltage at point 40 (called V2) frominput 290. This voltage passes through buffer 23 to electrode 37 which,in an exemplary embodiment, is a working electrode. The voltage V2 canbe measured by the microcontroller 21 via line 28.

The other electrode 38 of the cell 22 is connected by switches 33, 34,35 to a reference voltage VREF (from input 36) at point 41 and to anoperational amplifier 31. The voltage at point 41 can be measured by themicrocontroller via line 32.

Depending on the positions of switches 33, 34, 35, the amplifier 31 isable to serve as a voltmeter or an ammeter. When it serves as an ammeterit is measuring the current through electrode 38 and thus through thereaction cell 22, and it gives rise to a voltage at point 42 that isindicative of the current. When it serves as a voltmeter it is measuringthe voltage at electrode 38, and this gives rise to a voltage at point42 that is indicative of the voltage. In either case, themicrocontroller 21 is able, via line 29, to measure the voltage at point42. Low-pass filter 30 is provided.

As a first step, the microcontroller 21 measures the voltage at thecounter electrode 38. This measurement is relative to the workingelectrode 37, meaning that the microcontroller 21 will need to measurethe voltages on lines 28 and 29 nearly contemporaneously.

It will be appreciated that both of the operational amplifiers 23, 31are on the same chip. Thus to a first approximation the offsets andtemperature drifts for the two op amps are likely to be about the same.

Next the microcontroller 21 guesses at a PWM duty cycle that may giverise to a desired voltage at the working electrode 37. (The choice of aninitial duty cycle may be preconfigured in the microcontroller firmwareor may be based upon past experience.) The duty cycle is applied andtime is allowed to pass so that the PWM filtered voltage is stable.

Next the microcontroller 21 measures the voltage at 40 again. If thevoltage at 40 is higher or lower than desired, then in a recursive waythe PWM duty cycle is adjusted to come closer to the desired voltage at40.

This cycle may be repeated several times.

In the case where the apparatus is being used to analyze a bodily fluidor other analyte, this sequence takes place:

Before the analyte has been introduced into the cell, V2 (the voltage at40) is calibrated. The voltage V1 (the voltage at 41) is monitored.

Next the analyte is introduced into the cell 22. The microcontroller 21performs the calibration again. It monitors V2. It monitors V1. Themicrocontroller 21 measures the output of the second op amp 31. In thisway analytical measurements are carried out with respect to the analytein the cell 22.

This sequence of events may be carried out as described in copendingU.S. application Ser. No. 10/907,790, which application is herebyincorporated herein by reference for all purposes.

An exemplary sequence of steps will now be described in greater detail.These steps make the following assumptions.

The offset is assumed to be stable after the calibration sequence.

The offset of the two amplifiers is assumed to be the same because theyare on the same chip and are under the same conditions.

The potential at the working electrode 37 is assumed to be the voltageat 28 plus the offset.

The potential at the counter electrode 38 is assumed (during sampleintroduction, recalibration, and amperometry) to be the same as thevoltage at 32.

The potential at the counter electrode 38 is assumed (duringpotentiometry) to be the same as the voltage at 29, minus the offset.

Calibration. During a calibration phase, switch 35 is on and switches 33and 34 are off. The PWM is adjusted so that the desired applied voltageis developed. Eventually the voltage at 40 is stable. Themicrocontroller also monitors any changes in the voltage at 32, andmeasures the voltage at 29. The difference between the two is themeasured offset within amplifier 31. The assumption is then made thatthe offset within amplifier 23 is the same or nearly the same.

Sample introduction. Next the system is readied for introduction of thesample in the cell 22. Switches 33, 35 are on and switch 34 is off. Whencurrent flows, this is an indication that the sample has beenintroduced. In an exemplary embodiment the sample is human blood (or areference solution for calibration) and the cell 22 contains a glucoseoxidase.

Recalibration. During this phase the switches remain as previously set.The PWM is adjusted as needed to give rise to the desired appliedvoltage, defined as the difference between the voltages at 37 and 38.The voltage at 28 should be stable. Changes in the voltage at 32 aremonitored as this affects the applied voltage. The current through thecell 22 is measured (by noting the voltage at 29).

Amperometry. For the amperometry phase, the switches are as before. ThePWM monitors the voltages at 32 and 28 to ensure that the appliedvoltage at the cell 22 is at the desired level. The current through thecell 22 is measured as before.

Potentiometry. Switches 33, 35 are turned off. Switch 34 is turned on.The system measures the potential difference between the working andcounter electrodes 37, 38 (the cell voltage), by measuring the voltagesat 29 and 28. The difference between those two voltages (plus two timesthe offset) is the measure of the cell voltage.

This approach uses inexpensive components and thus helps to minimizecost.

Those skilled in the art will have no difficulty devising myriad obviousimprovements and variations upon the embodiments of the inventionwithout departing from the invention, all of which are intended to beencompassed by the claims which follow.

1. A method for use with a digitally controlled power supply and areaction cell having first and second load electrodes and a liquidsample disposed between the load electrodes, the power supply coupledwith the first load electrode to provide an excitation voltage to saidfirst load electrode, a first signal line coupling the first loadelectrode with a first analog-to-digital converter, and a voltage sensorcoupled with the second load electrode, the voltage sensor having asecond signal line coupled to a second analog-to-digital converter, themethod comprising the steps which follow in the order given: making afirst approximation to a digital control signal for the power supply;measuring a first potential at the second load electrode by means of thesecond signal line; making a second approximation to the digital controlsignal for the power supply, wherein the second approximation differsfrom the first approximation based on the difference between the firstpotential measured at the second load electrode and a target value;measuring a second potential at the second load electrode by means ofthe second signal line; making a third approximation to the digitalcontrol signal to the power supply, wherein the third approximationdiffers from the second approximation based on the difference betweenthe second potential measured at the second load electrode and thetarget value and measuring a measurement potential at the first loadelectrode by means of the first signal line, said measurement potentialbeing used for analysis of the liquid sample.
 2. The method of claim 1wherein the power supply is a pulse-width-modulated power supply and thesteps of making approximations to digital control signals each comprisesmaking an approximation to a duty cycle for pulse-width-modulating thecontrol signal.